The invention relates to a method and apparatus for printing characters which can be easily erased for correction purposes.
For all known printing systems, the need has long been felt for the possibility of erasing faulty printed characters. Such a need has not been satisfied completely up to now, when inks are used for printing. The mechanical removal of the ink from the paper by means of abrasive erasers and of discoloring chemicals give a far from perfect erasure while considerably damaging the paper.
Such inconvenients are only partly overcome by the so called lift off systems, that require a perfect match of the writing and erasing media, and by the cover up systems that leave very visible erasures and in any case present the problem of the unavoidable color differences between the white covering material and the paper.
There are known some systems where the printing can be erased by means of heating. In one known arrangement a laser beam is used to erase the characters to be deleted. With ordinary printing inks, this requires the use of a high power laser source. Apart from the cost of such an eraser, the temperatures required to cause the disappearance of the ink marks from the paper are such that they cause the burning or darkening of the paper.